You already know that verbs which speak of a direct object use the accusative case:

Aš perku knygą. I am buying a book.

However, when this verb is negative (with the prefix ne), the accusative case must change into the genitive case:

Aš neperku knygos. I am not buying a book.

Jis duoda Vytautui pieštuką. He gives a pencil to Vytautas.
Jis neduoda Vytautui pieštuko. He does not give a pencil to Vytautas.